1. Technical Field
The present invention relates to a multipath signal judging method and others for judging whether a receiving signal is a multipath signal or not.
2. Related Art
GPS (global positioning system) is widely known as positioning system using positioning signals, and applied to positioning device contained in cellular phone, car navigation system and the like. According to the GPS, the current position of the positioning device is determined through positioning calculation which obtains four parameters of three-dimensional coordinates showing the position of the positioning device and time error based on information about positions of plural GPS satellites and pseudo distances between the respective GPS satellites and the positioning device.
Multipath is one of the chief factors which cause errors in position measurement using measuring signals. The environment where multipath is generated is referred to as multipath environment. In the multipath environment, multipath signals produced by superimposing indirect waves such as reflection waves reflected by buildings, ground or the like, transmissive waves transmitted through obstacles, and diffraction waves diffracted through obstacles on direct waves transmitted from a signal generating source (GPS satellites in case of GPS) are received. Under this environment, indirect waves become error signals, making it difficult to decode codes.
Various technologies for reducing the effect of multipath have been proposed. For example, a method disclosed in JP-A-2000-312163 performs correlation calculation between a reception signal and plural replica codes of C/A codes having different phases for each chip. Then, the phase range where the correlation value is expected to become the peak value is calculated. Subsequently, correlation calculation between the reception signal and plural replica codes having different phases for each 0.1 chip is performed to detect code phase.
However, it is generally difficult to judge whether the reception signal is multipath signal or not. Thus, a so-called preventive method is usually employed which reduces the effect on the position measurement calculation without judging whether the reception signal is multipath signal or not even when the signal is multipath signal. For example, the technology disclosed in JP-A-2000-312163 is a method for detecting appropriate code phase even when the reception signal is multipath signal. In this structure, various disadvantages such as increase in the calculation volume and the circuit scale are provided when this preventive method is employed under the present situation.